Multiple sclerosis is the principle CNS demyelinating disease in humans. It is characterized by clearly demarcated plaques of myelin damage in association with inflammatory cell infiltrates. Work with models of experimental allergic encephalomyelitis (EAE) indicate that the induction of the disease involves class II restricted CD4+ T cells with specificity for defined epitopes of myelin basic protein or other neuroantigens. However, recent evidence indicates that while these neuroantigen-specific lymphocytes can orchestrate the influx of a large number of inflammatory cells into the CNS, the actual demyelination process depends on the action of inflammatory cells without apparent immune reactivity to neuroantigens. Our work addresses a potential mechanism by which these nonspecific cells interact with myelin. Our previous studies raise the possibility of a primary role for the lectin-like leukocyte receptor called L-selectin. Using antibodies and specific carbohydrates as inhibitors of L-selectin and employing a soluble recombinant form of L-selectin to identify ligand sites, we have shown that L-selectin can mediate the in vitro binding of lymphocytes to myelinated tracts of the CNS. In the present application, we plan to investigate the pathologic significance of what is most likely a fortuitous interaction between L-selectin and carbohydrate-bearing ligands within myelin membranes. Specifically, we will ask: l) Does L- selectin on leukocytes cause the targeting of the these cells to myelin? and 2) Does myelin damage then ensue from a process of cell-mediated toxicity or from the local release of cytokines such as TNF-alpha and lymphotoxin? Our planned experiments will consider whether blockade of L- selectin by antibodies will be sufficient to prevent or reduce clinical signs and demyelination in chronic relapsing models of EAE and whether a L-selectin knockout mouse is not susceptible to the passive transfer of the disease. Secondly, we will investigate whether signaling can occur through ligation of cell surface L-selectin and whether leukocyte adherence to myelin via L-selectin can trigger signaling. A final objective is to identify and characterize the myelin-associated ligand for L-selectin. Our experiments will investigate a novel mechanism by which CNS demyelination may occur. Our results may have broad significance for devising new therapies for intervention in a variety of CNS demyelinating conditions.